ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-12-337-2012Constraining the CO<sub>2</sub> budget of the corn belt: exploring uncertainties from the assumptions in a mesoscale inverse systemLauvauxT.1SchuhA. E.25UliaszM.5RichardsonS.1MilesN.1AndrewsA. E.4SweeneyC.4DiazL. I.1MartinsD.1ShepsonP. B.3DavisK. J.11Department of Meteorology, The Pennsylvania State University, Inversity Park, Pennsylvania, USA2NREL, Fort Collins, Colorado, USA3Purdue University, W. Lafayette, Indiana, USA4National Oceanic and Atmospheric Association, ESRL/GMD, Boulder, Colorado, USA5Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA05012012121337354This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/12/337/2012/acp-12-337-2012.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/337/2012/acp-12-337-2012.pdf

We performed an atmospheric inversion of the CO<sub>2</sub> fluxes over Iowa
and the surrounding states, from June to December 2007, at 20 km
resolution and weekly timescale. Eight concentration towers were used
to constrain the carbon balance in a 1000×1000 km<sup>2</sup> domain in this
agricultural region of the US upper midwest. The CO<sub>2</sub>
concentrations of the boundaries derived from CarbonTracker were
adjusted to match direct observations from aircraft profiles around
the domain. The regional carbon balance ends up with a sink of
183 Tg C±35 Tg C over the area for the period June–December,
2007. Potential bias from incorrect boundary conditions of about
0.55 ppm over the 7 months was corrected using mixing ratios from four
different aircraft profile sites operated at a weekly time scale,
acting as an additional source of uncertainty of 24 Tg C. We used two
different prior flux estimates, the SiBCrop model and the inverse flux
product from the CarbonTracker system. We show that inverse flux
estimates using both priors converge to similar posterior estimates
(20 Tg C difference), in our reference inversion, but some spatial
structures from the prior fluxes remain in the posterior fluxes,
revealing the importance of the prior flux resolution and distribution
despite the large amount of atmospheric data available. The retrieved
fluxes were compared to eddy flux towers in the corn and grassland
areas, revealing an improvement in the seasonal cycles between the two
compared to the prior fluxes, despite large absolute differences due
to representation errors. The uncertainty of 34 Tg C (or 34 g C m<sup>2</sup>)
was derived from the posterior uncertainty obtained with our reference
inversion of about 25 to 30 Tg C and from sensitivity tests of the
assumptions made in the inverse system, for a mean carbon balance over
the region of −183 Tg C, slightly weaker than the reference. Because of
the potential large bias (~24 Tg C in this case) due to choice of
background conditions, proportional to the surface but not to the
regional flux, this methodology seems limited to regions with a large
signal (sink or source), unless additional observations can be used to
constrain the boundary inflow.